Add experimental logic in GRExprEngine::EvalEagerlyAssume() to handle

expressions of the form: 'short x = (y != 10);' While we handle 'int x = (y !=
10)' lazily, the cast to another integer type currently loses the symbolic
constraint. Eager evaluation of the constraint causes the paths to bifurcate and
eagerly evaluate 'y != 10' to a constant of 1 or 0. This should address
<rdar://problem/6619921> until we have a better (more lazy approach) for
handling promotions/truncations of symbolic integer values.

llvm-svn: 65480